Preparation of particulate matter for freeze drying

ABSTRACT

A new method of preparing an extract for freeze drying has been discovered which produces particles of improved appearance in that they are more uniform and have a desirable shape and color. Preparation of frozen particles via the method of this invention is simplified and freeze drying efficiency is increased. The particles are prepared by slush freezing an extract (e.g. coffee extract) to a hard slush condition (as opposed to a soft slush condition) wherein the slush has an ice crystal content of from 20 to 60 percent by weight of the slush and the slush has a viscosity such that it can not be readily spread on a flat surface and will not tend to run or spread after extrusion, extruding the slush in a desired shape into a cold gaseous atmosphere having a temperature below the eutectic point of the extract, cutting the extruded slush and completely freezing the particles prior to freeze drying.

United States Patent Elerath [451 Jan. 25, 1972 [72] Inventor: ByronEverett Elerath, Mountain Lakes,

[73] Assignee: General Foods Corporation, White Plains,

[22] Filed: Jan. 14, 1969 [21] Appl. No.: 791,157

[52] US. Cl ..99/7l, 34/5, 62/270,

[51] Int. Cl. ..A23f 1108 [58] Field of Search ..99/65, 71, 72, 73, 192,193,

[56] References Cited UNITED STATES PATENTS 2,751,687 6/1956 Colton..34/5 $244,533 4/1966 Clinton et a1. 3,373,042 3/1968 Elerath et a1.3,408,919 1 H1968 Guggenheim 3,443,963 5/1969 Simon et a1 3,468,6729/1969 Schartzberg..... 3,449,129 6/1969 Reimus et al.

3,482,990 12/1969 Pfluger et a1 ..99/71 X 3,495,522 2/1970 Muller..99/236 2,292,447 8/1942 lrwin, Jr .99/199 X 2,431,496 11/1947 Natelsonet al. ..99/7l 2,509,681 5/1950 Flosdorf ....99/206 3,309,779 3/1967Ginnette et a1. ..34/5

3,419,399 12/1968 Earle, Jr. et al ..99/71 OTHER PUBLICATIONS Tea andCoffee Trade 1. 1953) Sept. page 23 Primary Examiner-Dennis E. Talbert,Jr. Attorney-Gerald E. Jacobs and Bruno P. Struzzi [5 7] ABSTRACT A newmethod of preparing an extract for freeze drying has been discoveredwhich produces particles of improved appearance in that they are moreuniform and have a desirable shape and color. Preparation of frozenparticles via the method of this invention is simplified and freezedrying efficiency is increased. The particles are prepared by slushfreezing an extract (e.g. coffee extract) to a hard slush condition 7Claims, No Drawings PREPARATION OF PARTICULATE MATTER FOR FREEZE DRYINGBACKGROUND OF THE INVENTION This invention relates to an improved methodfor freezing fruit and vegetable extracts preparatory for freeze drying.More particularly it concerns the formation of an extrudable slush whichcan be readily cut into particulate pieces suitable for use in a freezedrying system. As used herein, cutting refers to any means ofsubdividing the extruded shapes such as slicing or grinding.

Extracts as used herein refer to a water solution of soluble solidsextracted from fruits, vegetables and other plant products. Typicalextracts would be orange juice and coffee percolate.

In US. Pat. No. 3,373,042 issued Mar. 12, 1968 a method of freezingcoffee extract was disclosed wherein an aroma bearing extract wasconductively cooled to below its ice point in a first cooling zone topartially freeze water to an extent wherein said extract would retainits shape and form on standing, said partially frozen extract beingexposed to a second conductive cooling zone to completely chill theextract below its eutectic point, thus completing the freezing step. Incommonly assigned pending application, Ser. No. 763,345 filed on Sept.27, 1968, it was disclosed that extract could be slushed and finallyfrozen by means other than conductive cooling. In the work reported thusfar, while a slush was formed in the first cooling zone which couldretain a preformed shape, the slush was a soft slush which could bespread out on some type of belt in the second cooling zone in order tocomplete the freezing. Belt freezing is the most common method offreezing now practiced in commercial freeze drying plants wherein theproduct to be freeze dried is an extract. While this method of freezinghas proven commercially feasible it has many disadvantages, e.g., highequipment costs, the need for cumbersome equipment which occupies alarge portion of the manufacturing plant and constant supervision byprocessing personnel. Thus, it may be readily seen that the slushproduced in the prior art is most readily adaptable to a belt freezingsystem which significantly adds to the complexity and cost of a freezedrying installation.

SUMMARY It has now been discovered that a fruit or vegetable extract canbe prepared in the form of a hard slush and that the hard slush can beextruded in the form of ribbons which retain their shape and can beeasily frozen before or after cutting without the necessity of usingbelt freezers. Typically the final freezing can be accomplished byextruding the ribbons into a cold gaseous atmosphere at a temperaturebelow the eutectic point of the extract. The gas used can be air butalso it may be an inert gas which may afford additional protection tovolatile aromatic constituents.

As used herein the term hard slush is intended to describe a mixture ofconcentrated extract and water ice wherein the ice has been frozen outof the original extract andthe viscosity of the slush formed is suchthat it cannot be readily spread on a flat surface (e.g., a freezingbelt). However, the hard slush can be extruded in ribbon form such thatthe ribbons will not tend to run or spread after extrusion.

The term ribbons as used herein is meant to describe any form such asropes, rods, bars, etc., which can be continually discharged from anextruder. One of the many advantages of this invention is that twodimensions of the final product can be uniformly formed on extrusion.The third dimension of the final particulate matter to be freeze driedis controlled by the means of cutting selected. The process of thisinvention eliminates the need for complex and costly belt freezingsystems.

DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of thisinvention is to continually freeze an extract containing to 45 percentsolids by weight of the extract in a continuous manner in order toproduce frozen particulate matter suitable for freeze drying. Acontinuous system is preferred so that the slush being formed (mixtureof extract and water ice) can be constantly agitated and moved in ahomogeneous mass through an extruder. It has also been found that smallice crystals are more desirable than large ice crystals in practicingthis invention, as with a given percentage of ice crystals in the slush,a harder slush is apparently obtained with the smaller ice crystals.Therefore, in practicing this invention a continuous scraped surfaceheat exchanger such as that manufactured by the Votator division ofChemtron has proven very satisfactory. I

The slush formed in the heat exchanger is forced through an extruder.The discharge end of the extruder is a die with openings which conformto the desired product shape. It is necessary in practicing thisinvention to produce a hard slush, such that it will retain its shapeafter extrusion and be very nearly in a completely solid condition. Ithas been found that the ice content may be varied from about 20 to 60percent by weight of the slush when the extract concentration variesfrom about 20 to 45 percent solids by weight of the initial extract. Thedesired degree of hardness will result from a combination of increasingviscosity due to the ice crystals being formed and the increasingviscosity of the concentrated extract which constitutes the liquid phaseof the slush. Normally the higher the initial solids content in theextract being frozen, the smaller the amount of ice which must becrystallized in order to form a hard slush.

The range of concentrations of 20 to 45 percent solids in the initialextract was chosen for a variety of reasons. The upper limit wasselected such that it is sufficiently below the eutectic composition ofthe extract to permit enough water to be crystallized out in the form ofice to obtain the necessary hard slush. The eutectic composition willvary for different extracts, but for fruit and vegetable extracts saidcomposition is generally above about 70 percent solids by weight of theex.- tract. Therefore, by limiting the extract concentration to an upperlimit of 45 percent, it is possible to freeze out enough water in theform of ice crystals such that the ice. content of the slush is at least20 percent. A lower limit of extract concentration of 20 percent wasselected in order to avoid having to freeze an excessive amount of icein order to obtain a hard slush. For most extracts, it is believed that60 percent ice in the slush will give a satisfactory hard slush whereinthe free liquid contains about 50 percent solids by weight of theextract. If one were to start with 20 percent extract and form a slushcontaining 60 percent ice, the solids concentration in the liquidportion of the slush would be 50 percent. Also, as a practical matter aninitial solids content of less than 20 percent would be undesirableeconomically since the object of this invention is to produce a productsuitable for freeze drying. Freeze drying is an expensive form ofdehydration and for most products it would be uneconomical to attempt tofreeze dry an extract containing less than about 20 percent solids.

The hard slush is extruded in the form of continuous ribbons which maybe out before or after additional freezing. A preferred technique is toslice the ribbons into desirable lengths as they are discharged from theextruder and to allow the pieces thus formed to fall through a cold airtunnel in order to completely freeze the pieces to below the eutecticpoint of the extract.

An alternative method would be to allow the ribbons to completely freezein a cold air stream and then pass the frozen ribbons through a cutteror grinder. This latter technique will produce particles with a morerandom size distribution and shape and may be desirable for someproducts. The shape of the pieces can also be varied by varying theshape of the openings in the extruder die as well as the size of theopenings in the die.

An additional advantage of this invention is that the texture and colorof the particle surfaces can be varied. This can be done by allowing asurface thawing of the ribbons in the extruder die head. On suchproducts as coffee extract this surface thawing can result in a darkercolor. The surface appearance can also be changed by plating the ribbonsof slush as they are extruded. A water spray for example would tend toquickly freeze on the surface of the ribbons and prevent potentiallytacky particles from sticking to each other. An extract spray would tendto have the same advantages as the water spray and in addition caneffect the color and appearance of the dry particles. Alternatively, theslightly tacky surface of the extruded ribbons could be coated with dryproduct in order to achieve unique surface effects.

This invention also offers the possibility of several processingadvantages in freeze drying. For example, the shape of the die openingcan be irregular in order to achieve particles with a maximum surfacearea. The greater surface area should result in an increased drying rateand thus increase the productivity of a freeze dryer. Also, the shape ofthe extruded pieces could be varied to obtain a final dried product witha specific, desired density. Density control may also be achieved byfoaming the extract prior to forming the slush or by foaming the hardslush prior to extrusion.

This invention will be further described by reference to the followingexamples.

EXAMPLE I Coffee extract containing 25 percent solids by weight of theextract was chilled in a continuous scraped surface, heat exchanger. Theresultant slush was chilled to a temperature of F. prior to extrusion.This slush was found to be a hard slush suitable for extrusion andcontained 44 percent ice by weight of the slush. The liquid portion ofthe slush had a concentration of 44.5 percent solids by weight of theliquid. The slush was extruded in the form of I [16-inch rods into anatmosphere of 30 F. air. The rods were cut with an oscillating wire toproduce l/16 to i 's-inch long pieces. These pieces were thoroughlyfrozen to 30 F. and collected in a hopper. The frozen pieces were thenloaded into trays and freeze dried at a vacuum of less than 500 micronsof mercury to a moisture content of 2.5 percent. The resultant productwas very regular in appearance, easily handled in packing operation, andessentially free of dust.

EXAMPLE ll Coffee extract containing percent solids was freezeconcentrated to produce an extract containing 35 percent solids byweight of the extract. The concentrated extract was then treated as inexample l. The slush was chilled to a temperature of 15 F. and theresultant hard slush contained 30 percent ice crystals by weight of theslush. The solids concentration of the liquid portion of the slush was50 percent.

It should be noted that a hard slush in this example was attained withonly 30 percent ice as compared to 44 percent ice in example I becausethe solids concentration of the initial extract was greater.

The resultant dry product was again found to be very uniform and easilyhandled in packing operations.

The foregoing examples are for illustrative purposes only and the scopeof the invention is to be limited only by the appended claims.

I claim:

I. A method of preparing cofiee extract for freeze drying comprisingchilling the extract to below 32 F. but above its eutectic point, saidextract having from 20 to 45 percent solids by weight of the extract;further chilling said extract to form a hard slush having an ice crystalcontent of from 20 percent to 60 percent by weight of the slush andhaving a viscosity such that it cannot be readily spread on a flatsurface; extruding said hard slush into a cold gaseous atmosphere havinga temperature below the eutectic point of the extract in the form ofcontinuous ribbons of desired dimension, which ribbons will retain theirshape and will not tend to run or spread after extrusion; cutting theextruded ribbons of hard slush and freezing the extruded hard slush tobelow the eutectic temperature of the initial extract.

2. The method of claim 1 wherein the ribbons are sliced into desiredlengths as they are discharged from the extruder.

3. The method of claim 1 wherein the extruded ribbons of hard slush areplated as they are extruded, the plating material being selected fromthe group consisting of water, coffee extract and dried coffee extract.

4. The method of claim 3 wherein the extruded ribbons are sprayed withwater as they are extruded.

S. The method of claim 3 wherein the extruded ribbons are sprayed withcoffee extract as they are extruded.

6. The method of claim 3 wherein the extruded ribbons are coated withpreviously dried coffee extract as they are extruded.

7. The method of claim 1 wherein gas is introduced into the extract toform a foam.

2. The method of claim 1 wherein the ribbons are sliced into desiredlengths as they are discharged from the extruder.
 3. The method of claim1 wherein the extruded ribbons of hard slush are plated as they areextruded, the plating material being selected from the group consistingof water, coffee extract and dried coffee extract.
 4. The method ofclaim 3 wherein the extruded ribbons are sprayed with water as they areextruded.
 5. The method of claim 3 wherein the extruded ribbons aresprayed with coffee extract as they are extruded.
 6. The method of claim3 wherein the extruded ribbons are coated with previously dried coffeeextract as they are extruded.
 7. The method of claim 1 wherein gas isintroduced into the extract to form a foam.